1. Field of Invention
This invention relates to a vane and shroud stator, which is integrally cast, having spring means for damping the unit.
2. Description of the Prior Art
Axial flow gas turbine compressors have been constructed of alternate sets of rotors and stators of a quantity and specific design as needed to meet the flow and pressure requirements of an engine cycle. The stators are constructed of an annular array of identical airfoils, or vanes, supported at one or both ends by circular rings. These rings, in addition to structurally supporting the airfoils, or vanes, also provide flowpath boundaries, and the inner rings, if used, often serve to support seal lands. These rings are often referred to as "shrouds". Stators constructed as a fabrication are assembled by attaching the required number of airfoils, or vanes, to the inner and outer shrouds. A number of optional attachment features exist including riveting, brazing, welding or staking the vanes to the shrouds at one or both ends. A fabricated stator assembly is shown in U.S. Pat. No. 4,285,633.
Due to vibratory excitation of the vanes by the turbulence and periodic perturbations in the airflow across the vanes, the vanes will vibrate at one or more of their natural frequencies. This vibration produces stresses and deflections in the vanes that can produce fatigue and wear. When a vane attachment to the shroud ring allows relative motion between these parts, beneficial friction damping is introduced. This friction damping is desirable in that it dissipates vibratory energy in the system and reduces vibratory stress and deflections improving vane durability. However, since the rubbing can produce wear, it may be undesirable in the long term if the wear progresses at a rate that is not consistent with the life requirements of the stator assembly.
To alleviate the expense and complexity of fabricated stator designs, entire stator assemblies are being cast as one piece, with the vanes and shrouds becoming integral homogenous elements of the entire stator. Although the cast construction improves cost and provides simplicity and consistency, its one-piece construction eliminates the relative motion between the vanes and the shrouds and therefore eliminates the built-in friction damping of the fabricated stator assembly.
A cast stator, which is an integral unit, will vibrate in a different manner than a fabricated assembly. Since the vanes are integral with the shrouds, vane vibratory motion is shared with one or both shrouds in a more complex motion than that which occurs in a stator fabricated of vane and shroud components. In the cast, integral, stator, bending and torsional modes of vibration of the vanes occur in concert with various elaborate nodal diameter patterns of vibration in the shrouds. The total vibration energy input to the system is coupled between the vanes and the shrouds.